This invention relates to a method of and apparatus for producing aerated cementitious compositions.
In particular, although not exclusively, the present invention relates to the production of lightweight aeerated cementitious compositions for filling or partly filling voids or cavities in underground mines.
In underground mines voids or cavities of varied and indeterminate size frequently occur as a result of mining operations. For example, cavities generally occur around roof supports installed in mine roadways or above roof supports installed along a working face. To promote efficient working of the underground mine the cavities are usually filled or partly filled with materials such as timber, broken rock or pumped cement grout. Unfortunately, difficulties are frequently experienced when filling cavities with these materials, for example, the infilling of cavities on a working face with timber can be potentially hazardous operation as there is a danger of rocks falling off the sides of a cavity and falling on the operators below. Cement grouts have specific gravities of above 1.0 and because of this they are especially difficult to contain within overhead cavities particularly when the available shuttering is typically of rudimentary nature. Substantial quantities of cement are required to fill large size cavities.
Also it is known for aerated or foamed cement compositions to be used in the manufacture of lightweight building blocks, such compositions being produced by one of two alternative methods. The foaming agent used may include one or more of the following materials: hydrolysed proteins, fatty acid salts, alkyl-aryl sulphonates, alkyl sulphates, phenol ethoxylates, the purpose of the foaming agent is to entrap air in a mix of foaming agent, cement and water. Foaming agents generally are based on materials which will lower surface tension of the composition and which help to maintain stability of air bubbles by slightly increasing the viscosity of the composition and by forming a stabilised skin to the bubbles. In one known method the foaming agent is added to the cement and water and the whole rigourously mixed. The foaming agent introduces and stabilises air bubbles during the mixing process. In an alternative method a stiff foam is first prepared usually in a form tube or via a venturi. The foamer device intimately mixes compressed air and the foaming agent solution to produce a continuous stream of stiff foam composed of small air bubbles. The stiff foam then is mixed with cement grout to produce an aerated cement grout. In both the above known methods fine or coarse aggregates may be introduced to the aerated cement grout.
Unfortunately, both the above prior known methods have disadvantages. With regard to the first disclosed method it is difficult to entrap sufficient air through a "whisking" technique to achieve both a desired density and a thixotropic characteristic. In addition, it takes a considerable period of time to whisk air into a mixture of cement grout and foaming agent. Moreover, this first technique does not lend itself to continuous operation and thereby obviates this method being used with the desirable very rapid setting cement compositions. With regard to the second disclosed method, it is a complicated manufacturing technique and therefore very difficult to operate in underground mining conditions. Also it is dependent upon two separate components, i.e., the foaming agent and the cement which must be mixed on site during the production of the aerated composition. Furthermore, it can result in an unhomogenous aerated composition being produced.